ASTRONOMICAL PHENOMENA AND PROGRESS. 



49 



Celestial Photography. The advance made in 

 this department of science has been so great as 

 to mark an era in astronomical progress. It is 

 now many years since the first successful ef- 

 forts to obtain by the photographic process 

 exact delineations of the mountains, craters, 

 landscapes, etc., on the moon, and spots and 

 f aculsa on the sun, were made ; but attempts to 

 photograph the stars, especially those which 

 are telescopic, invariably resulted in failure. 

 Now, however, persistent and long-continued 

 effort united with skill has overcome all ob- 

 stacles, and success surpassing the most san- 

 guine anticipation hns at last been achieved. 

 Stars as faint as the fifteenth magnitude are 

 now depicted on the sensitive plate, giving an 

 absolutely perfect record and a chart of the 

 heavens which in point of accuracy is unap- 

 proachable by human hands. 



The following is a translation of the report 

 of the Henry brothers, who are connected with 

 the Observatory of Paris, descriptive of the in- 

 strument by means of which these wonderful 

 pictures are obtained : " The instrument con- 

 sists of two object-glasses placed side by side 

 in a hexagonal tube, their optical axes being 

 parallel and separated by a thin partition 

 through its entire length. One of the ob- 

 ject-glasses (9 inches aperture and 11 feet fo- 

 cal length) is corrected for visual rays, and 

 serves as a finder. The other (aperture 13 - 4 

 inches and focal length 11 feet 3 inches) is cor- 

 rected for actinic rays and is used for photogra- 

 phy, and covers a field three degrees in diame- 

 ter." The instrument is mounted equatorially 

 and driven by clock-work, running for three 

 hours without being re-wound, and has very 

 tine .independent back movements. The pho- 

 tographic objective is the largest yet made, and 

 was constructed by the brothers Henry. 



On the 23d of April last an exposure of one 

 hour gave a fine negative, which on a surface 

 ten inches square, representing five square de- 

 grees of the heavens, showed distinctly 2,790 

 stars from the fifth to the fourteenth magni- 

 tudes. Those at the edges of the plate were 

 as sharply defined as those in its center. The 

 stars of the fourteenth magnitude had a diame- 

 ter of about -^Vo f an inch. Traces of stars 

 of the fifteenth magnitude can be seen on the 

 negative, but are too feeble to be reproduced 

 on the sensitized paper. By longer exposure 

 not only they but sixteenth-magnitude stars 

 can certainly be obtained. The construction 

 of such a chart made in one hour would, by 

 ordinary methods, require several months of 

 assiduous labor. 



The following are the necessary minimum 

 exposures; though, to obtain negatives for good 

 paper prints, triple the time is required : 



1st magnitude stars require 0'005 second. 



2d magnitude stars require 0'013 second. 



3d magnitude stars require 0'03 second. 



4th magnitude stars require O'OS second. 



5th magnitude stars require 0'2 second. 



6th * magnitude stars require 0'5 second. 



* Faintest visible to naked eye. 

 A 



7th magnitude stars require 1*8 second. 



8th magnitude stars require 3-D seconds. 



9th magnitude stars require 8*0 seconds. 



10th magnitude stars require 20-0 seconds. 



llth magnitude stars require 50'0 seconds. 



12th magnitude stars require 2 minutes. 



13th magnitude stars require 6 minutes. 



14th magnitude stars require 13 minutes. 



15th magnitude stars require 83 minutes. 



16th magnitude stars require 1 hour and 23 minutes. 



The table shows that the difference of ex- 

 posure varies between stars of the first and 

 the sixteenth magnitudes as 1 to 1,000,000. 

 Between two consecutive magnitudes the rela- 

 tion is 2*512. The plates used were the gela- 

 tine-bromide. The well-known cluster in Per- 

 seus was photographed by these gentlemen 

 in fifty minutes. The negative plate shows 

 stars down to the thirteenth magnitude 509 

 in all. The telescope employed was a 6'3-mch 

 refractor. After enlarging the photograph four 

 times, it was reproduced by the photo-engrav- 

 ing process, and embodied in the " Annual Re- 

 port of the Paris Observatory." These copies 

 are on the scale of five inches to a degree. 



Mr. Common, of England, who is authority 

 on this subject, says : " Any star that can be 

 seen at all with a telescope can be photo- 

 graphed with the same, and, furthermore, stars 

 that can not be seen at all will, by long expos- 

 ure, reveal themselves on a negative plate. 

 These facts are on the border-line of the mar- 

 velous, and must work a revolution in obser- 

 vational astronomy. Some one has well said, 

 ' A library may now bo made, not of books full 

 of figures and descriptions the accumulated 

 work of many men working many years, each 

 on his own system but of pictures written on 

 leaves of glass by the stars themselves.' " 



As there are about 41,000 superficial square 

 degrees in the firmament, and as each negative 

 represents five such degrees, it follows that 

 about 5,000 pictures would include the entire 

 sky, with not one of the 20,000,000 stars, not 

 even those of the fifteenth magnitude, missing. 



Eclipses. The regions within which the an- 

 nular phase of the eclipse of March 16 was 

 visible were mostly upon the ocean and in an 

 uninhabited part of the earth. And, though 

 the eclipse was annular along a line passing 

 through the northern part of California and 

 the Northwestern Territories, yet no extensive 

 arrangements for its observation were made. 

 The scientific interest attaching to an annular 

 eclipse is, however, of little importance com- 

 pared with that of a total one, for, except for 

 noting the times of the several contacts for the 

 determination of the moon's diameter and of 

 the inequalities of her motions, annular eclipses 

 possess little scientific value. Not so with a 

 total eclipse. For, while his disk is covered 

 by the moon, which sometimes lasts for several 

 minutes, time is had for valuable observations 

 telescopic, spectroscopic, and polariscopic 

 of the sun's surroundings, especially of the red 

 prominences on the sun's limb, and of the co- 

 rona which completely and, in certain direc- 

 tions, extensively surrounds him. 



